Coated catalysts and various processes for producing them are known.
Usually, catalyst supports of various shapes are impregnated with a liquid composition which is enriched in a catalytically active material and/or a precursor thereof.
During impregnation of the catalyst supports with the liquid composition, the catalytically active components and/or the precursors thereof not only get onto the surface of the catalyst supports but also into fine channels of the porous structure of the support material. An example of an impregnation method is dry impregnation (“incipient wetness” method). Here, a porous support material is impregnated with a liquid composition which contains just that amount of liquid which corresponds at most to the total pore volume of the support material. After the liquid composition has been taken into the pores of the support material, the latter appears dry.
When reactants are used or when reaction products are produced, it is often impossible or barely possible for these to diffuse toward or away from the catalytically active component. Catalytically active components in narrow open pores of the support material in this case do not participate, or participate to only a limited extent, in the reaction or the long residence time in the pores resulting from diffusion induces undesirable secondary reactions. It is therefore desirable to deposit the costly active component as far as possible on the surface or in the vicinity of the surface of the support material, with, in the broader sense, a shell of active component consisting of only a few atomic layers being formed around the support material or in the uppermost layers of the support material or individual accumulations of particles of active component being formed on the surface or in the upper layers of the surface of the support material.
Various methods of producing coated catalysts are known from the literature. In typical spray impregnations, for example as described in EP 0 599 193 A1, catalyst supports are placed in a stirred vessel, mixed by rotation of the stirred vessel and sprayed with the impregnation solution in such a way that the liquid phase of the solution can evaporate quickly. In this way, relatively large batches of coated catalysts can be spray-impregnated with respectable uniformity.
Further methods of applying catalytically active species to a catalyst support are known from EP 2 420 317 A1, EP 0 672 450 A1, DE 28 14 262 A1, DE 197 34 974 A1 and DE 197 34 975 A1.
DE 102 33 864 A1 discloses a process and an apparatus for producing pulverulent products, in which liquid active materials are atomized and applied to a support material. Here, the active material is conveyed together with an atomization medium present under superatmospheric pressure in a liquid, supercritical or gaseous state in separate product streams to a multifluid nozzle and the atomization medium is subsequently depressurized and the active material is very finely atomized. The atomized active material is subsequently brought to a support material. The apparatus is proposed, in particular, for applications in food technology and pharmacy.
U.S. Pat. No. 4,858,552 A1 describes an apparatus for pelletizing particles. Here, a nozzle can be used for introducing a plurality of media into a fluidized bed. Not only liquid components but also gaseous components can be introduced into the nozzle so as to exit concentrically around a liquid stream and assist atomization of the liquid.
In the processes known hitherto for producing coated catalysts, precursor solutions consisting of only one metal component were generally used. When a plurality of metal components were necessary, a consecutive mode of operation was generally pursued; i.e. impregnation with the second component was carried out after impregnation with the first component. The use of multimetal precursor solutions was generally not possible because of the lack of stability of such solutions. Further deficiencies of previously known systems for spraying liquid compositions are the absence of preheating of the propellant gas and the absence of preheating of the composition to be sprayed. Likewise, little attention was paid to the widening of the spray jet when using an ultrasonic expansion nozzle, even though the use of such nozzles has been described in EP 0 599 193 A1.